According to WHO statistics, as many as 170 million people worldwide are infected by hepatitis C virus (HCV), a viral infection of the liver. 75 to 85% of persons infected with HCV progress to chronic infection, approximately 20% of these cases develop complications of chronic hepatitis C, including cirrhosis of the liver or hepatocellular carcinoma after 20 years of infection. The current recommended treatment for HCV infections is a combination of interferon and ribavirin drugs, however the treatment is not effective in all cases and the liver transplantation is indicated in hepatitis C-related end-stage liver disease. At present, there is no vaccine available to prevent HCV infection, therefore all precautions to avoid infection must be taken.
Thus, patient care, as well as the prevention of transmission of Hepatitis C Virus (HCV) by blood and blood products or by close personal contact requires extreme vigilance using sensitive detection assays. This creates a need for specific methods for screening and identifying carriers of HCV and HCV-contaminated blood or blood products. Serological determination of HCV exposure relies on the detection of HCV present in human blood plasma or sera. This can be accomplished by detection of distinct structural and non-structural proteins encoded by the virus.
The HCV virus is a (+) sense single-stranded enveloped RNA virus in the Hepacivirus genus of the Flaviviridae family. The viral genome is approximately 10 kb in length and encodes a 3011 amino acid polyprotein precursor. The HCV genome has a large single open reading frame (ORF) coding for a unique polyprotein. This polyprotein is co- and post-translationally processed by cellular and viral proteases into three structural proteins, i.e., core, E1 and E2 and at least six non-structural NS2, NS3, NS4A, NS4B, NS5A and NS5B proteins. (Choo et al., Science 244: 359-362 (1989)).
Following HCV exposure, the virus enters a susceptible hepatocyte and viral replication occurs. During an eclipse phase period of approximately 10 days, viral presence is not evident (i.e., viral RNA cannot be detected), serum transaminase levels are within normal limits, and no evidence exists of an immune response to HCV (Busch et al., Transfusion 40:143 (2000)). Typically, about 10 days following exposure, HCV RNA can be detected, often with viral loads between 100,000-120,000,000 HCV RNA copies per ml of serum. Typically several weeks later, an increase in ALT levels is observed, indicating inflammation of the liver; antibodies are detected an average of about 70 days after exposure.
Screening of blood for exposure to HCV, either by the detection of antibodies to HCV or by the detection of viral-specific molecules (e.g., HCV RNA or HCV core proteins) in serum/plasma is an integral and important part of patient care. Blood or blood products derived from individuals identified as having been exposed to HCV, by these tests, are removed from the blood supply and are not utilized for distribution to recipients of blood products (see, e.g., U.S. Pat. No. 6,172,189). These tests may also be utilized in the clinical setting to diagnose liver disease attributable to HCV infection.
Serologic antibody tests rely on the use of recombinant antigens or synthetic peptides, representing selected fragments of the viral polyprotein. The first generation anti-HCV screening tests were based on detection of antibodies directed against a recombinant protein (HCV genotype 1a) originating from sequences located in the nonstructural NS-4 protein (C100-3) (Choo et al., Science 244:359 (1989); Kuo et al., Science 244:362 (1989)). The first generation assays failed to detect antibodies in approximately 10% of individuals having chronic HCV infection and up to 10-30% of individuals presenting with acute HCV infection. The second generation anti-HCV assays have incorporated recombinant proteins from three different regions of the HCV genome (HCV genotype 1a), including amino acid sequences from the core, NS3, and NS4 protein (Mimms et al., Lancet 336:1590 (1990); Bresters et al., Vox Sang 62:213 (1992)), allowing a marked improvement over the first generation tests in identifying HCV infected blood donors (Aach et al., N Engl J Med 325:1325 (1991); Kleinman et al., Transfusion 32:805 (1992). The second-generation assays detect antibodies in close to 100% of chronic HCV cases (Nino K., Intervirology 37:77 (1994)) and in nearly 100% of the acute cases by 12 weeks post infection (Alter et al., N Engl J Med 327:1899 (1992); Bresters et al., Vox Sang 62:213 (1992)). The third generation test includes a recombinant protein expressing amino acid sequences from the NS5 region, as well as antigens from the core, NS3 and NS4. Some studies have indicated a slight improvement in sensitivity in comparing the third generation tests to second generation tests (Lee et al., Transfusion 35:845 (1995); Courouce et al. Transfusion 34:790-795 (1994)), but this improvement is largely attributed to changes in the NS3 protein rather than the inclusion of NS5 (Courouce et al., Lancet 343:853 (1994)).
In general, the second and third generation HCV antibody tests detect exposure to HCV about 70 days after exposure. Since HCV establishes persistent, and in many cases lifelong infection, the detection of antibodies to HCV represents a very efficient method for determining exposure to HCV. However, antibody testing alone will frequently fail to detect HCV infected individuals during the first 70 days after exposure.
It has been suggested that testing for HCV antigen detects exposure to HCV significantly earlier than antibody testing and represents an alternative to nucleic acid testing for detecting exposure to HCV during the pre-seroconversion period. The HCV antigen detection test is rapid, simple, may-not require sample extraction or other pretreatment, and is not as prone to handling errors (e.g., contamination) as may occur in the HCV RNA tests. Thus, HCV core antigen tests present a practical alternative to HCV RNA for screening blood donors or for monitoring antiviral therapy.
Existing HCV antigen tests rely on detecting the presence of the HCV core antigen in serum or plasma. HCV core protein is a structural protein of HCV comprising the first 191 amino acids of the polyprotein and that forms the internal viral coat encapsidating the genomic RNA. Two different types of serologic assays have been developed which permit detection of HCV core antigens in serum. One assay format detects HCV core antigens in subjects prior to seroconversion and is utilized in screening blood donors, while the other assay format detects core antigens only in hepatitis C patients, regardless of their HCV antibody status, and is utilized in clinical laboratories to diagnose exposure to HCV or to monitor antiviral therapy. The currently available core antigen detection assays all use antibodies against the DNA binding domain of HCV core which is located at amino acids 1-125 of the core protein. The core protein also contains a lipid binding domain that is located between amino acids 134-171. To date there have been no antigens described from that section of core protein and until now it has been assumed that core detection required antibodies against the DNA binding domain.
Thus, binding proteins that can readily detect HCV core antigen will markedly improve the available methods of detection of HCV exposure in a patient. Thus, there is a recognized need for new antibodies that can readily be employed in screening tests.